The Kingdom of Breifne or Bréifne (anglicized Breffni, Breffny, Brefnie, Brenny) is a confederation of Irish túaithe headed by a ruirí drawn from the Uí Briúin Bréifne.

The Bréifne territory includes current day Irish counties of Leitrim and Cavan, along with parts of County Sligo (an area roughly equivalent to the modern Roman Catholic Diocese of Kilmore).

At its height in the 1twelfth century, when Tigernán Ua Ruairc is king of Bréifne, it extends from Kells in County Meath to Drumcliffe in County Sligo.

In 1296, according to the Annals of the Four Masters, "Maelpeter O'Duigennan, Archdeacon of Breifny...died."

In 1256, a great battle is fought between the O'Rourkes and the O'Reillys near Ballinamore.

This leads to the division of Breifne between the O'Rourkes and O'Reillys.

The Bréifne region is split into East Bréifne and West Bréifne.

The Ó Ruairc kings (O'Rourke) maintains lordship over West Bréifne (mainly County Leitrim), while the Ó Raghallaigh kings (O'Reilly) retained lordship over East Bréifne (County Cavan).

The Kingdom of Bréifne region is part of the kingdom of Connacht up until the time of Queen Elizabeth I.

In that time it is shired into the modern counties Cavan and Leitrim, Leitrim remaining a part of the province of Connacht while Cavan becomes part of Ulster.

The Ó Ruaircs are effectively lords of Breifne O'Rourke through the turbulent sixteenth century.